The production of acids and bases from their salts may be achieved using an electrically driven process including a bipolar membrane and an aqueous medium. The process is represented by the equation: ##EQU1##
To effect and maintain separation of the various species, ion exchange membranes are used. The most crucial of these membranes is the bipolar membrane, so called because it is composed of regions that are selective to ions of opposite charges. Under the influence of an applied direct current, such a sandwich membrane is capable of forcibly dissociating water to form equivalent amounts of hydrogen and hydroxyl ions. Used in conjunction with other cation-selective and anion-selective (i.e., monopolar) membranes, the assembly constitutes a water splitting apparatus that generates acid and base.
The basic structure of bipolar membranes is shown in FIG. 1. Bipolar membranes include a cation selective layer, an anion selective layer and a thin interfacial region where the two ion exchange layers are in contact. The interfacial region may be either a plane or surface of contact or a layer itself, perhaps including adhesives or resin layers that help bring the cation and anion exchange layers into contact. In typical bipolar membranes, the cation selective layer, the anion selective layer and the interfacial region will be continuous and of essentially uniform thickness. The interfacial plane or layer is very important because it is where reactions, such as water or salt splitting reactions, take place under the influence of an applied electric field. However, before the reactions can take place, the species being split must move from outside the membrane through either the cation or anion exchange layers and into the interfacial region. Bipolar membranes of this or similar construction have been used as diaphragms in the electrolysis of water to hydrogen and oxygen or as separating membranes used in reclaiming acids and alkalis from aqueous solutions of salts.
Bipolar membranes behave anisotropically under the influence of an electric field as is illustrated by the transport process shown in FIG. 1. When a current is passed across the bipolar A membrane with its cation selective side facing toward the anode and a salt solution disposed on either side of the bipolar membrane, cations and anions are transported to the interfacial region through the permselective membranes. However, the passage of ions out of the interfacial region is limited since the ions may not pass through regions of the bipolar membrane having the wrong selectivity. Consequently, salt builds up at the interfacial region and causes a low electrical resistance in the interfacial region. When the orientation of the membrane is reversed, as shown in FIG. 2 with the cation selective side facing toward the cathode, and current is passed, salt from the interfacial region is transported to the external solution leaving only H.sup.+ and OH.sup.- ions from the dissociation of water to carry the current. With this orientation of the membrane, the electrical resistance of the interfacial region can become high since water has a low conductivity. Alternatively, bipolar membranes may be used in accordance with FIG. 2 to split water and salt. If the interfacial region is made very thin, the resistance of the interfacial region-can be small and the membrane may be used to generate acid and base.
One of the most important parameters for the design of processes for electrodialysis of an acid and base is the electrical resistance exhibited by the membrane. Significant limitations are placed on the amount of current that can be applied across a bipolar membrane. For example, in water splitting, H.sup.+ and OH.sup.- ions generated at the interface or interfacial region of the bipolar membrane require water for their formation. This water must diffuse to the interfacial region through either the cation selective layer or the anion selective layer. In addition, as the ions move to the base and acid compartments on either side of the bipolar membrane, the ions remove additional water from the membrane as water of hydration. As the current density is increased, the rate of water removal at the interface and throughout the membrane is increased. If the transport of water into the interfacial region, through either or both of the cation- and anion-selective layers from the adjacent solution, is not as rapid as the removal of water away from the interfacial region, by consumption and water of hydration, then some points of the interfacial region will dry out causing the water splitting process to slow down. Dry spots in the bipolar membrane will cause an even higher current density over that portion of the interfacial region that remains hydrated. Furthermore, the drying out of the interfacial region which results when the current density is too high can lead to irreversible damage to the membrane which manifests itself in still higher electrical resistance. This in turn increases the amount of energy required to drive the process.
One exemplary method for producing bipolar membranes discloses that a cation-exchange membrane and an anion-exchange membrane can be laminated using a mixture of polyethyleneimine and epichlorohydrin to bond the membranes to each other by curing (Japanese Patent Publication No. 32-3962). A second method discloses the bonding of a cation-exchange membrane to an anion-exchange membrane by using an adhesive having the properties of exchanging ions (Japanese Patent Publication No. 3403961). A third method discloses a pasty material comprising vinyl pyridine and an epoxy compound that is coated on the surface of a cation-exchange membrane, followed by exposure to radiation to obtain the bipolar membrane (Japanese Patent Publication No. 338-16633). A fourth method discloses a sulfonic acid polymeric electrolyte and an allylamine that are adhered to the surface of an anion-exchange membrane, followed by exposure to ionizing radiation (Japanese Patent Publication No. 51-4113). Yet another method discloses a process in which a polyethylene film is impregnated with styrene and divinylbenzene followed by polymerization to give a sheet-like material. The sheet-like material is then nipped between frames made of stainless steel, where one side thereof is sulfonated, and thereafter, the sheet is detached and the remaining side is chloromethylated followed by treatment for amination (U.S. Pat. No. 3,562,139). However, these bipolar membranes exhibit inherently poor current efficiency and high-power consumption. For example, the use of these bipolar membranes to split water requires application of a membrane potential (e.g. 2.5 V to 3.0 V, or higher) that is much higher than the theoretical water-splitting membrane potential (0.83 V).
Bipolar membranes have also been prepared by coating the mating surfaces of cation-and anion-exchange membranes with a solution comprising at least one kind of inorganic electrolyte, such as sodium tungstate, chromium nitrate, sodium metasilicate, and ruthenium trichloride. The mating surfaces are placed in contact and pressed to give a bipolar membrane having a low water-splitting membrane potential. This bipolar membrane, compared with the bipolar membranes discussed above, is characteristic of a low water-splitting membrane potential. However, the water-splitting potential of this membrane increases over a relatively short period of use due to the development of bubbles or blisters at the interface between the cation-exchange membrane and anion-exchange membrane. Partially or entirely separated membranes are rendered useless. Furthermore, this membrane still provides low current efficiency and is not satisfactory for use on an industrial scale.
Bipolar membranes which are used, for example, for isolating acids or bases from their salts by electrodialysis are ion exchange membranes having fixed cations on one side and corresponding anions on the other side. Bipolar membranes may be produced, for example, by firmly anchoring cationic or anionic groups to both sides of a neutral membrane by means of a chemical treatment (U.S. Pat. No. 4,0557,481) or bringing an anion-exchange membrane into close contact with a cation-exchange membrane, for example by pressing the membranes on top of one another in the presence of heat (British Pat. No. 1,038,777).
Attempts have been made to increase the stability of such bipolar membranes, obtained by combining anion-exchange membranes with cation-exchange membranes, by applying an ion-permeable adhesive between the two membranes. A polymerizable mixture of polyethyleneimine and epichlorohydrin (U.S. Pat. No. 2,829,095) or polyvinyl chloride and polyvinyl alcohol (Israel Journal of Chemistry, 9(1971),485) has been proposed as an adhesive. It has also been found that bipolar membranes suitable for electrodialysis may be obtained from an anion-exchange membrane, a cation-exchange membrane and an ion-permeable adhesive comprising of an aqueous solution of a polyvinylamine.
Bipolar membranes are difficult to produce by conventional methods. For example, in a chemical treatment of the surface, the two layers must have uniform thickness and must be in contact with one another over the entire surface area in order to ensure the current flow. On the other hand, the layers must not penetrate one another since the membrane would then lose its bipolar selectivity. Although combining two monopolar membranes gives bipolar membranes possessing defined anionic and cationic layers, this method gives rise to difficulties at the contact surface. If the membranes are not completely in contact, the resistance increases. The same applies where the adhesive is not sufficiently conductive. Moreover, very undesirable tears or bubbles may form at the points of contact in bipolar membranes of the stated type under typical operating conditions.
Bipolar membranes which consist of two individual membranes and polyvinyl alcohol as an adhesive may be prepared by a method in which the cation-exchange films and the anion-exchange films are coated with a polyvinyl alcohol solution. The cation-exchange and anion-exchange films are laid one on top of the other and heated for about one hour at about 60.degree. C. The bipolar membrane is then dried and compressed for about 30 minutes at about 100.degree. C. Although the resulting bipolar membranes exhibit firm adhesion, their swellability in aqueous salt solutions is irreversibly restricted, and these bipolar membranes, which possess rectifying properties, are therefore unsuitable for electrodialysis.
Despite certain advances described above, the performance of bipolar membranes is still limited by the transport of water into the interfacial region. U.S. Pat. No. 4,851,100 proposes to increase water transport to the interfacial region by using a continuous layer of a cation-selective material that is sufficiently thin to reduce the distance the water must diffuse to reach the interfacial region. This bipolar membrane is made by affixing a thin castable cation exchange membrane to a defined base anion exchange membrane. While a bipolar membrane of this construction might provide some increase in the rate of water transport to the interfacial region, the water transport rate is still limited because it has to pass through an ion-selective layer.
Therefore, there is a need for a bipolar membrane that provides for improved communication of fluids to the interfacial region between the cation-selective layer and the anion-selective layer. It would be desirable if the bipolar membrane provided direct communication of fluids to the interfacial region. It would be further desirable if the bipolar member were stable and exhibited low electrical resistance.